This invention generally relates to measuring submicron structures or shapes; and more specifically, the invention relates to measuring such shapes from electron beam images of the shapes. Even more specifically, the present invention relates to a measurement procedure and system particularly well suited for determining the relative position of edges of submicron structures or shapes formed by photolithography on semiconductor wafers.
As photolithography K factors continue to decrease and complex reticle enhancement techniques are employed, the printed shapes appearing on the wafer can vary dramatically from the design in both shape and location. The traditional metrics of line width and overlay, although still useful, often are inadequate for predicting many effects on product. A means of accurately measuring wafer images in a full two dimensions is required for a variety of applications including fully understanding effects of imaging, etching, and other wafer processing on product performance. Another especially important application of two-dimensional shape metrology is for use in the calibration of shape prediction models that are just becoming widely offered in the semiconductor industry.
Little has been done in the area of full two-dimensional submicron shape metrology. Early two dimensional submicron measurements are generally achieved with a top down SEM. Metrology SEMs are generally one-dimensional instruments, that scan in a single direction that is approximately normal to the edge to be detected. An edge detection algorithm is then applied to this signal. Early attempts at two-dimensional metrology with SEM's consist of multiple one-dimensional measurements (e.g. to evaluate a bar, a scan along the width will be followed by a different scan along the length). This treatment of shapes is generally inadequate for many applications in terms of quantity of data and it is customary for the scans to be independent (i.e. the two scans are not connected by a coordinate system with a single origin).